Understory plants are a major contribution to the high plant species diversity characteristic of many Neotropical rain forests. Shrubs, understory trees, saplings of over-story trees, and large-leaved herbs occupy a habitat of generally low light levels and high humidity in which there seem to be few obvious mechanisms to support habitat partitioning. Moreover several plant families are characterized by a high number of co-occurring understory species (e.g., Melastomataceae, Piperaceae, Rubiaceae). We explored factors contributing to the species diversity of plants in Neotropical rain forests by sampling understory vegetation in 18 sites at the La Selva Biological Station of the Organization for Tropical Studies in the Caribbean lowland rain forest of Costa Rica. At each site we used 20 nested quadrats to investigate the effects of soil type (5 alluvial and residual volcanic mapped soil units) and topographic position (ridges, midslopes and flats) on composition, density and diversity of small (1 m tall to 5 cm dbh, 25 m2 quadrat) and large (5-10 cm dbh, 100 m2 quadrat) understory plants. We also measured fine litter dry mass, pH, extractable P, total organic matter, percent slope and canopy openness at 1 m at each quadrat.
Results/Conclusions
Non-metric Multidimensional Scaling based on 18 stands and 273 species (small plants) and 141 species (large plants) demonstrated consistent patterns in composition among sites with respect to the mapped soil units and topographic position. Stand composition differed between the two residual soil units for small but not large plants; for both groups stand composition differed between alluvial and residual soil types and among stand topographic positions. These patterns were correlated with percent slope, extractable P, total organic matter, fine litter mass, and canopy openness. Plant density and species diversity were significantly correlated for both small and large plants. Small but not large plants were significantly more dense on slopes than on flats and ridge tops. Both groups were more species rich on slopes. Percent slope was also a factor in the distribution of species within single plant families. We suggest soil characteristics as well as processes promoting high plant densities, such as high stand turnover rates, contribute to the maintenance of species diversity in rain forests.